Heat-treatable, high-strength, high-toughness, low-carbon, ni-mo alloy steel

ABSTRACT

An alloy steel consisting of about 0.15-0.25 percent carbon, 812 percent nickel, 0.50-1.5 percent molybdenum, up to 1 percent each of manganese and silicon, up to 0.25 percent V, balance substantially all iron, heat treatable to a combination of high strength and high toughness with controlled austenitic transformation.

United States Patent Savas 1 1 Feb. 29, 1972 [54] HEAT-TREATABLE, HIGH-STRENGTH, 3,444,011 5/1969 Nagashima ..75/123 X HIGI-LTOUGHNESS, LQW.CARB()N, N]- 3,366,471 1/1968 Hill et al. ..75/123 M0 ALLOY STEEL FORElGN PATENTS OR APPLICATIONS [721 lnvenw" Bmksville, 1,079,036 8/1967 Great Britain ..75/123 K [73] Asslgneez gilirgbllc Steel Corporation, Cleveland, OTHER PUBLICATIONS Brophy, G. R., and A. J. Miller, The Metallography and Heat [22] Flled' 1968 Treatment of 8 to 10% Nickel Steel, ln Trans. of American [21] App]. No.: 700,440 Society for Metals, Vol. 41, 1949, pp. 1 185- 1203.

Primary Examiner-L. Dewayne Rutledge [52] US. Cl ..75/l23 J, 75/123 K, 148/36 Assistant Examinehl' 5 Les, [5]] Int.'Cl. ..C22c 39/36, C22c 39/50, C22C 41/00 Ammey Robe1-t p wright d Joseph w Maneck [58] Field of Search"; ..75/l23, 126,128,123 K I [57] ABSTRACT [56] References Cited An alloy steel consisting of about 0.15-0.25 percent carbon, UNITED STATES PATENTS 8-12 percent nickel, 0.501.5 percent molybdenum, up to 1 percent each of manganese and silicon, up to 0.25 percent V, 3,496,034 2/1970 Alger ..75/l28 R balance substantially n iron heat treatable to a combination 2244'064 6/1941 HodgeW" X of high strength and high toughness with controlled austenitic 2,337,049 12/1943 Jackson ....75/123 X transformation 2,451,469 10/1948 Brophy ....75/123 X 2,992,148 7/1961 Yeo ..75/123 X 4 Claims, 2 Drawing Figures HEAT-TREATABLE, HIGH-STRENGTH, HIGH- TOUGHNESS, LOW-CARBON, NI-MO ALLOY STEEL This invention pertains to an alloy steel of novel composition, which on austenitizing at elevated temperature, followed by quenching and tempering is characterized by high strength and toughness, and by a microstructure consisting preponderantly of martensite but containing a controlled amount of austenite.

An object of the invention is to provide a low-carbon alloy steel containing nickel and molybdenum as essential constituents and in proportions such that a controlled proportion of austenite is retained in the steel after quenching, thus to impart increased toughness to the steel along with high strength.

A further object of the invention is to provide an alloy steel having a composition which displays an optimum combination of high toughness and strength when heat treated as aforesaid.

Great effort has been directed within recent years to development of alloy steels suitable for applications in the various advanced technological fields, such as in aerospace, atomic power, submersibles and cryogenics. In each application, certain unique properties or combinations of properties are required in order to meet specific rigidly set conditions imposed upon the steel.

For example, one of the properties which is required in some of the foregoing applications includes the ability to retain strength and toughness up to about l,000 F.

Another requirement is that a steel designed for cryogenic use must be capable of withstanding extremely low or subzero temperatures without a substantial reduction in strength or toughness. Additionally, in the field of atomic energy and in particular, atomic power vessels which contain the atomic reactor unit, most stringent requirements are imposed on the structural steel employed, including the ability to retain its strength and toughness under constant bombardment by radioactive emission.

A number of such high strength steels, i.e., of 180 k.s.i. yield-strength or better, have recently been developed for various of the newly emerging applications. These appear to fall generally within two categories, (a) the maraging steels and (b) the quenched and tempered steels.

The maraging steels have found commercial acceptance because of their excellent mechanical properties developed without necessity for quenching. These steels, however, are expensive because of their relatively high-alloy content. The present invention is concerned with steels of relatively lowalloy content and of improved composition, in the quenched and tempered category.

The latter class of steels, however, have been plagued with problems of achieving a desired combination of high strength and toughness. Generally when a steel is quenched and thereafter tempered, the structure is susceptible to becoming brittle in the process. To overcome the problem of embrittlement, it has been attempted to lower the tempering temperature even down to 400 F. It has been found, however, that by thus lowering the tempering temperature, the toughness of the steel is unduly sacrificed. Therefore, although the low alloy, quenchable and temperable steels can be heat treated to extremely high-yield and tensile strengths, the requisite toughness characteristics have heretofore been lacking for structural applications.

ln addition, most high-strength steels, in an effort to arrive at some optimization of the combined properties of strength and toughness, have resulted in an overall higher cost and hence have been confronted with an economic barrier to wide, successful utilization.

In no instance insofar as I am aware, has an alloy steel been produced having the combination of high strength and high toughness exhibited by the steel of the present invention, as suitably heat treated, and which additionally is relatively economical to produce.

1n the steel of the present invention, nickel and molybdenum are essential constituents, in amounts of about 8-12 percent nickel and 0.50 to 1.50 percent molybdenum,

Weight Percent of Element Total Alloy Carbon 0.15-0.25 Nickel 8.0l2.0 Molybdenum 0.50-1.50 Manganese 0l.00 Silicon 0-1 .00 Vanadium 0-0.25 Sulphur 0.020 max. Phosphorus 0.020 max.

lron Balance substantially Other constituents, either singly or in combination, have been, or can be, used in the steel of the present invention in very minor amounts and may replace all or a portion of the manganese and silicon. Such additional ingredients may comprise chromium, cobalt and certain other deoxidizing elements such as aluminum, titanium or zirconium.

Within the above broad composition range, there is a preferred range for the elements which produce a steel having excellent combinations of toughness and strength. The preferred range is as follows: about 0.18 to 0.22 percent of carbon, about 9.8 to 10.2 percent nickel and about 1.10 to 1.20 percent molybdenum, and 0.01 to 0.15 percent each of Mn and Si.

The steel of the present invention is in general, subjected to normalizing at a temperature within the range of about 1,625 to 1,650 F. for a period of about 1 hour. Thereafter it is allowed to air-cool to room temperature. The steel may then be austenitized at a temperature above 1,500 F. and thereafter oil-quenched with sufficient rapidity that substantially all of the austenite is converted to martensite. Some austenite, however, in the amount of about 2-5 percent of the quenched microstructure is retained in the steel after quenching to room temperature, and for purposes as hereinafter explained.

The quenched steel may then be tempered over a wide range of temperature which may typically be between about 400 to l,000 F. for a time of about 4 hours. Additional heat treatments may be utilized either prior or subsequent to the tempering, depending on particular desired characteristics of the steel. For instance, low-temperature treatment such as refrigeration at about 1 20 F., may be used, followed by tempering as above.

As an example of a typical steel made in accordance with the present invention and which as heat treated displays excellent high-toughness and strength properties, it contains about 0.20 percent carbon, 0.11 percent manganese, 0.10 percent silicon, 9.9 percent nickel and 1.15 percent molybdenum. For developing these properties the steel is normalized for 1 hour at a temperature of l,625 F., air cooled to room temperature, austenitized for 1 hour at 1,525 F., oil quenched and then tempered for 4 hours at l,000 F. As thus heat treated the steel has a 0.2 percent ofiset yield strength of 172 k.s.i. (thousands of pounds per square inch), an ultimate tensile strength of 181 k.s.i., a Charpy V-notch impact energy value of 82 fL-lbs. at room temperature and 68 ft.-lbs. at -l00 F.

It has been found that the foregoing composition and others within the range heretofore indicated, produce a final alloy steel which, upon heat treatment, retains a portion thereof in the form of austenite. That is to say, a controlled transformation has taken place that in a certain predictable amount of austenite is retained in the alloy steel composition after quenching to room temperature from the austenitizing temperature. Yet the proper quenching of the present steel substantially avoids the other undesirable austenite transformation products, such as bainite and pearlite. This is accomplished by quenching at a sufficiently rapid rate to follow a path on a time-transformation-temperature curve such that the cooling takes place to the left of the curve, thereby permitting of quenching directly'to martensite. The resultant steel alloy is therefore, substantially martensitic. However, we have found that by carefully selecting the composition of the treated steel, a portion of the alloy additionally will be retained austenite.

The prior art has for some time, sought means to obtain an exceptionally high-toughness and high-strength steel and more recently has turned to the use of nickel, cobalt, and chromium either separately or in combination, along with various other alloying additions in order to achieve these results. All such attempts, however, have resulted in various drawbacks in one respect or another.

In accordance with the present invention, a low-alloy, lowcarbon steel has been developed having relatively few alloying elements, and yet which achieves an optimum combination of strength and toughness. A feature of the invention is the development of a low-carbon steel alloy wherein some of the.

additives, heretofore thought necessary, have been eliminated without encountering a corresponding deleterious effect on the alloy steel properties. As an example, steel produced in accordance with the present invention is capable of displaying combined high-strength and toughness characteristics without the addition of cobalt, which addition is undesirable in applications of such a steel for atomic power reactors.

It has been found, in accordance with one aspect of this invention, that the ductility of an otherwise martensitic alloy is increased, with retention of high-tensile strength, by formulat-,

ing a composition having a nickel content in the critical range of about 9.5l0.5 percent. By the selection of this nickel content in the steel of the invention, the temperature at which the formation of martensite commences (M,) is lowered, and thereby the transformation of austenite to martensite is controlled on cooling to assure the retention of a selected amount. As a consequence of this, a controlled amount of austenite is retained after quenching, and the quenched alloy exhibits favorable characteristics of ductility as well as the high-tensile strensthglgrmally aa ie s wit ma nefi twgee After quenching, the steel may be further toughened by subsequent tempering operations. The tempering cycles may include multiple tempering steps. As an example, the steel may be normalized at l,625 F. for 1 hour followed by air-cooling to room temperature, then austenitized at l,525 F. for 1 hour. After austenitizing, the steel is oil quenched, again to room temperature, followed by subsequent tempering, preferably at about 1,000 E, although acceptable results may be obtained by tempering at temperatures as low as about 400 F. Tempering durations are generally up to about 4 hours.

In the accompanying drawings:

FIG. 1 is a graphical chart showing the relationship between room temperature yield strength and Charpy V-notch (CVN) impact energy values for steels of the compositions shown, as austenitized, quenched and thence tempered at l,0O0 F.

FIG. 2 is a similar graphical chart in which the steels have been tempered ata temperature of 400 F.

The te st data plotted in FIGS. 1 and 2, is based on the test results given in Table ll below for steels of the compositions given in the following Table l.

TABLEI Chemical analysis of various high nickel vacuum induction melted alloys Heat Analysis No. C Mn Si Ni Co Cr Mo V Al V459 0.14 0.45 0.00 9.6 2.0 0.14 0.40 0.16 0.01 vszs 0.23 1.33 0.20 9.0 2.0 1.290.05 V524 0.24 0.16 9.9 1.2a V561 0.11 0.11 0.11 10.1 0.19 0.43 0.04 V568 0.20 0.11 0.10 9.9 1.15 V569 0.18 0.11 0.10 10.5 1.21 V576 0.19 1.00 0.15 10.6 1.1a

Of the above compositions, V568, V569 and V576 are steels according to the present invention while the remaining steels are not, including extraneous elements such as Co, Cr, etc., and/or excess Mn.

TABLE 11 [Mechanical properties of various vacuum induction melted alloys one-half inch thick plate product in various heat treated conditions] Charpy V-Notch (IL-lb.) Test Y.S T.S. RA. Elong. eat Heat Number dir. (Ks.i.) (Ks.i.) percent percent R, F 0 F. treat.

V459 T 169 183 67 18 42 56 62 A T 163 196 63 15 40. 5 51 64 B T 127 191 61 16 41 57 C V523 L D T D L E T E L F T F T G V524 L D T D L E T E L F T F T G V567 L D T D L E T E L F T F T G V668 L" D T D L E T E L F T F Tl G [Mechanical properties of various vacuum induction melted alloys one-half inch thick plate product in various heat treated conditions] Charpy V-Noteh (IL-lb.) Test Y.S. T.S. R.A. Elong. Heat Heat Number dlr. (15.1.) (Ks.i.) percent percent R 100 F 70 F. treat.

Ves) L D T D L E T E L F T F T G V576 L D T D L E 'I E L F '1 F '1 G "11 out treatments as follows:

A=Norm,1,650(1), AC,Aust.l,550(1), 0Q, '1 400(2), R (-120) (4), T 400(2).

B=Norm. 1,6500), AU, Aust. 1,5500), 0Q, DT1.000(2 2) C=Norm.1,650(1), AC. D=Norm. 1,6250), AC, Aust. 1,6250), 0Q, '1 400(4). AC, Aust. 1,5250), 0Q, T 800(4).

0Q, T 1,000(4). G=Norm. 1,6250), AC. L=Longitudinal to rolling direction. T=Transverse to rolling direction. "Indicates optimum strength-toughness properties.

lileflsap les exhibit25% retai ed austentite as examined by X-ray difiraction techniques.

Referring to the test results of Table I1, and more particularly to such as are graphically plotted in FIG. 1 for the steels as tempered at 1,000 F it will be seen that heats V568, V569 and V576 according to the invention have by far the best com-' binations of strength and toughness as compared to the others. Thus all of the heats according to the invention had 0.2 percent offset yield strengths in excess of 170,000 k.s.i. and Charpy impact values within the range of 60-90 ft.-lbs. at 70 F. As compared to this, of the remaining steels tested those having equal or higher impact strengths, had substantially lower yield strengths, such as heats V567 and V459, while those having higher yield strengths had much lower impact strengths, such as heats V524 and V523.

Referring to FIG. 2 for the steels as tempered at 400 F it will be seen that there is in general a loss of yield strength with no improvement in impact strength.

The test results further show that optimum strength and toughness is found where the alloy composition has the least amount of extraneous alloying elements. The test data thus shows that elimination of certain alloying elements, such as cobalt, chromium, vanadium and aluminum increases the austenite stability and enhances the optimum combination of strength and toughness in the steel alloy.

As further shown, the effect of such additional alloying elements tended to decrease either the toughness or the strength of the steel alloy. Also as shown, the effect of increasing the amount of manganese from 0.10 percent to 1.1 percent raised the overall yield strength of the alloy by only 1,000 p.s.i. (1 k.s.i.) but caused the toughness to decrease by some 19 ft.-lbs.

The increase of carbon content caused a similar decrease in the toughness but a relatively small increase in the yield strength. This is illustrated by a comparison of heat V576 having a carbon content of 0.19 percent with heat V524 having a carbon content of 0.24 percent and a higher manganese percentage. Although, in this comparison, the higher carbon heat V524 caused an increase in yield strength of 7,000 p.s.i., it was accompanied by a decrease in the impact toughness of 23 ft.- lbs.

Tuming now to the effect of cobalt and vanadium, the test results for heat V524 containing no cobalt or vanadium, may be contrasted thh a .2 omm nal-Q aassaitsela t and 0.05 percent vanadium, from which it is seen that while the overall yield strength is raised, the impact toughnws experiences a proportionate decrease in value.

It is apparent, therefore, from the foregoing analyses and by comparisons of the melts of various compositions, that an optimum range of values for the components of the steel of the present invention lie within the values heretofore presented.

in addition, the subsequent heat treatment of the steel determines its eventual characteristics and, in particular, the temperature at which tempering is carried out is an important factor in the final values.

Referring again to Table 11 and FIGS. 1 and 2, it may be seen generally that the best combination of improved strength and toughness is achieved by tempering at the higher tempering temperatures. In the particular examples of heats V568 and V569, the steels which were tempered at the temperature of 1,000" P. experienced the greatest combination of strength and toughness.

What is claimed is:

1. An alloy steel consisting of about: 0.15 to 0.25 percent carbon, 8 to 12 percent nickel, 0.5 to 1.5 percent molybdenum, up to 1 percent manganese, up to 1 percent silicon, up to 0.25 percent vanadium, and the balance all iron, characterized as normalized at about 1,625 to 1,650 F., thence austenitized above 1,500 F., oil quenched and tempered at about 4001,000 F., by a yield strength of at least 170,000 p.s.i., a Charpy V-notch impact strength of at least 60 ft.-lbs. at 70 F. and a substantially martensitic microstructure.

2. A heat-treated alloy steel according to claim 1 having a microstructure consisting substantially of martensite and about 2-5 percent austenite in the quenched condition.

3. A heat-treated alloy steel according to claim 1 having a preponderantly martensitic microstructure having a room temperature yield strength of at least k.s.i. and a Charpy V-notch impact strength of at least 60 ft.-lbs. at 70 F.

4. An alloy steel according to claim 1 containing about: 0.18 to 0.22 percent carbon, 9.8 to 10.2 percent nickel, 1.1 to 1.2 percent molybdenum, 0.01 to 0.15 percent manganese, 0.01 to 0.15 percent silicon. 

2. A heat-treated alloy steel according to claim 1 having a microstructure consisting substantially of martensite and about 2-5 percent austenite in the quenched condition.
 3. A heat-treated alloy steel according to claim 1 having a preponderantly martensitic microstructure having a room temperature yield strength of at least 170 k.s.i. and a Charpy V-notch impact strength of at least 60 ft.-lbs. at 70* F.
 4. An alloy steel according to claim 1 containing about: 0.18 to 0.22 percent carbon, 9.8 to 10.2 percent nickel, 1.1 to 1.2 percent molybdenum, 0.01 to 0.15 percent manganese, 0.01 to 0.15 percent silicon. 